Quantitation of metabolic and radiobiological effects of 6-aminonicotinamide in RIF-1 tumor cells in vitro.

6-Aminonicotinamide (6AN) can be metabolized to 6-amino-NAD(P+), a competitive inhibitor of NAD(P+)-requiring processes, especially the pentose phosphate pathway (PPP) enzyme, 6-phosphogluconate dehydrogenase. The effect of 6AN on the flux of 1 and 6 13C-labeled glucose to lactate, via glycolysis and the PPP, was investigated using 1H-nuclear magnetic resonance. These studies showed that 6AN as a single agent caused a significant 89% (P < 0.0001) inhibition of glycolytic flux but had no detectable effect on the PPP. 31P-nuclear magnetic resonance studies of perifused RIF-1 cells indicated that 4 h of exposure to 6AN were sufficient to cause significant accumulation of 6-phosphogluconate, the substrate for this enzyme (P < 0.0001). A significant reduction in the phosphocreatine: inorganic phosphate ratio was observed under conditions that led to accumulation of 6-phosphogluconate (P < 0.006). Accumulation of 6-phosphogluconate and subsequent reduction in phosphocreatine correlated with significant potentiation of 6 Gy of irradiation by 6AN. These results suggest that the radiation enhancement effect of 6AN may be due to inhibition of glycolysis (mediated by 6-phosphogluconate) and the associated reduction in high-energy phosphates. Additional studies analyzing the metabolic effects of 6AN in combination with radiation are necessary to determine the role of inhibition of the PPP in 6AN enhancement of radiation.